It is a constant challenge to assess the contents of cargo containers using non-intrusive methods. The desire to do so is often fueled by the need to detect contraband and the threats posed, for example, by nuclear weapons. There is a need, therefore, to detect high-Z materials characteristic of shielded radioactive sources and unshielded/shielded special nuclear materials.
Radiation Portal Monitors (RPMs) can detect unshielded or lightly shielded radioactive materials in lightly loaded cargos. However, RPMs cannot detect radioactive sources shielded by high-Z materials, or partially shielded radioactive sources and Uranium-235 in medium-to-heavy cargo. High-energy X-ray inspection systems are widely deployed to detect general contraband and more recently, they have been used to detect shielded and unshielded nuclear materials in cargo.
Current research includes employing Muon Tomography (MT) as a primary inspection system and method for detecting special nuclear materials and shielded radioactive materials. A muon is a charged particle with a mass of 206 times that of the electron with a charge of −1. It has a lifetime of around 6 microseconds, which gives it just enough time to get from the outer edge of the atmosphere, where they are created due to interaction of very energetic protons which arrive at the Earth after travelling billions of miles through deep space, to the surface of the Earth where we can use them for imaging.
Muons are minimum ionizing particles—they lose a bit of their energy by collision with atomic electrons. Thus, the muon travels in a substantially forward direction after each collision, but with a small deviation to the left or the right. After very many collisions, the amplitude of the total deflection from the original direction increases, and this angle of deflection is weakly dependent on the Z (atomic) number of the materials through which the muon is travelling. By measuring the direction of the muon entering a volume and the direction of the muon as it exits the volume, we can draw a line from the point of entrance in the correct direction into the volume and the point of exit in the direction back into the volume and the point of intersection between the two gives the effective center of the scattering material.
However, for detection using Muon Tomography (MT) as a primary inspection system, the throughput is low compared to that for X-ray systems (few minutes vs. tens of seconds). These systems require large detectors that extend the complete object under inspection, which makes the systems expensive and more complex. Also the systems do not provide high-resolution images used to detect general contraband by customs agencies.
Once a high-Z material is detected by a primary system, there is a need to confirm whether the material is fissile. The standard method used for confirmation is based on active interrogation, namely using neutron and/or high-energy sources to induce fission and array of gamma-ray and/or neutrons detectors to detect the radiation emitted from the fission process. However, these systems are complex, expensive, and produce high radiation requiring either a large exclusion zone or a shielded facility.
Therefore, there is a need for a secondary or second stage low-cost system to confirm the presence of shielding and nuclear materials with high confidence in a relatively short time, and with a small exclusion zone compatible with port environments, thereby enabling wide deployment.